The instant invention pertains to semiconductor device fabrication and processing and more specifically to a novel phase-shift lithography mask.
Presently, optical microlithography is the most-favored technology for the mass production of semiconductor devices. This technology involves printing a pattern or relief image on a recording medium, known as photoresist, using a projection camera as the pattern exposing tool. These cameras, known as steppers, scanners, or step-and-scanners, project a master pattern contained in a mask or reticle onto a wafer coated with a light sensitive material, known as photoresist. A problem with present optical microlithography methods is that they are diffraction limited. In other words, the smallest features that can be printed are limited by the diffraction of the light as it passes through the openings in the reticle. One method used to circumvent this problem involves phase shifting the light on the reticle 180 degrees so as to improve the contrast of the aerial image. Different types of phase shift methods have been proposed but problems involving mask making processes have limited the number of techniques that, have been introduced into production.
Typically, the mask (or reticle) is comprised of a piece of quartz on which the desired patterns are etched in chromium or other opaque material. The exposure light is blocked wherever there is the opaque material and the light is transmitted to the wafer in places devoid of the opaque material. Hence, the mask pattern is transferred to the wafer surface by the lithography process. Since only two different regions appear on the standard reticle (i.e. an opaque portion and a non-opaque portion), this type of reticle is called a binary reticle.
An attenuated phase shift mask, APSM, uses a special semi-transparent material which allows a certain amount of light to pass through. The transmission of this type of film is typically less than 14 percent. By adjusting the film thickness, the phase of the light transmitted through this semi-transmissive material can change by 180 degrees. In this manner, the destructive interference in terms of transmitted and semi-transmitted light can occur at the tail of the aerial image. This destructive interference can improve the contrast of the aerial image in addition to the resolution and the depth of focus.
A limitation of the APSM involves the printing of sidelobes. Due to diffraction, the amplitude of the electrical field can change from a positive global maximum to a negative local minimum. The first local minimum of the electrical field, or the first maximum in terms of light intensity (which is the square of the electric field), is the main contributor of sidelobe printing. Generally, other peaks in the electrical field distribution are too small to have a significant impact. If the first local maxima of light intensity from transparent features near the mask feature are close to one another on the wafer plane, or even worse, they have similar phase angles, the total light intensity, by coherent and incoherent constructive interference, can be higher than the resist develop threshold. This will result in the printing of sidelobes. Though this phenomenon does not usually occur if a binary mask is utilized, it is a serious problem for attenuated phase shift masks. This is especially true if the transmittance of the semi-transparent material is high. For attenuated phase shift masks, light transmitted through the semi-transparent materials will coherently or incoherently add to the overlap region, in addition to the first intensity maxima introduced by nearby open features, thereby making the sidelobe much more readily printed than with the binary mask. Sidelobes cause problems because an undesired opening may be formed in the layer to be patterned.
The attenuated phase shift mask of the instant invention is basically comprised of three components: an open area, a phase-shift area (preferably comprised of a semi-transmissive region), and an opaque area (preferably comprised of chromium). The phase-shift area is preferably situated between the open area and the opaque area. Preferably, the energy transmitted through the phase-shift area is around 180 degrees out of phase in comparison to the light transmitted through the open area. The transmittance of the mask of the instant invention can be quite high, whereas the transmittance in a conventional attenuated phase shift mask must be kept low so as to reduce the printing of the sidelobes.
An embodiment of the instant invention is a mask having a pattern which is transferred to a layer overlying a semiconductor wafer, the mask comprising: a transmissive portion, the transmissive portion allowing energy which impinges upon the transmission portion to substantially pass through the transmissive portion; a substantially non-transmissive portion; a semi-transmissive portion situated between the transmissive portion and the substantially non-transmissive portion, energy passing through the semi-transmissive portion having a phase; and wherein the phase of energy which passes through the semi-transmissive portion is out of phase with the phase of energy which passes through the transmissive portion. Preferably, the phase of the energy which passes through the semi-transmissive portion is around 180 degrees out of phase with energy which passes through the transmissive portion.
Another embodiment of the instant invention is a mask having a repeating pattern which is transferred to a layer overlying a semiconductor wafer so as to define a repeating feature over the semiconductor wafer, the mask comprising: a transmissive portion which allows energy impinging upon it to substantially pass through it, the energy passing through the transmissive portion of the mask having a phase; a substantially non-transmissive portion which substantially impedes energy impinging upon it to pass through; a semi-transmissive portion situated between the transmissive portion and the substantially non-transmissive portion of the mask, energy passing through the semi-transmissive portion having a phase which is different than the phase of energy passing through the transmissive portion of the mask; and wherein the transmissive portion has a shape selected from the group consisting of: rectangular, circular, oval, octagonal, square, and any combination thereof. Preferably, the phase of the energy passing through the semi-transmissive portion is around 180 degrees out of phase with the energy passing through transmissive portion. The substantially non-transmissive portion is, preferably, comprised of chromium. Preferably, the semi-transmissive portion is comprised of a material which allows between 2 and 18 percent (more preferably 6-12%xe2x80x94even more preferably 8-10%) of the impinging energy to pass through the semi-transmissive portion. The impinging energy is, preferably, comprised of: DUV energy, I-line energy, x-ray energy, electron beam energy, optical energy and any combination thereof, and the thickness of the semi-transmissive portion is, preferably, around one-half the wavelength of the impinging energy. Preferably, the semi-transmissive portion is concentric to the transmissive portion, and the edge of the semi-transmissive portion which abuts the substantially non-transmissive portion has the same shape as the edge of the semi-transmissive portion which abuts the transmissive portion.